Technical Field
The present disclosure relates to integrated circuits, and more particularly, to integrated circuits having an improved electromigration performance and a metal with an increased grain size, and a method of forming the same.
Related Art
Integrated circuit interconnects, and particularly, high performance conductors are used for most types of advanced integrated circuits and are typically fabricated having thick metal wires such as copper (Cu) or aluminum (Al). Traditionally, the metal wires are formed using electrolytic plating processes in conjunction with photoresist masking and stripping, and removing a seed layer later on. In damascene processes, metal interconnect lines are delineated and isolated in dielectrics by means of chemical mechanical polishing (CMP). A dual damascene process is a similar process in which metal interconnect lines and vias (i.e., conductor-filled channels) are defined independently in photolithography and etch but metallized simultaneously.
Conventionally, the damascene process includes forming an opening, e.g., a trench, in a dielectric layer on a substrate. The opening may be coated with a liner and/or seed layer. Subsequently, a metal, e.g., copper, is plated such that the metal substantially fills the opening. The metal is plated such that it includes a large number of grains which are much smaller than the wiring dimensions, e.g., linewidth. This large number of grains results in the interconnect having a larger resistance than a single-grained structure of the same dimensions. To increase grain size and reduce the resistance, the unannealed wiring structures may be held at room temperature for a period of the order of several hours to a few days in order to allow room-temperature recrystallization to occur. To reduce the overall time of the damascene process, a gentle furnace anneal may be performed to increase the grain size of the metal. To avoid exposing the interconnect to too much heat, the furnace anneal is typically performed at approximately 100° C. for about an hour. Conventional annealing at temperatures higher than about 100° C. results in faster grain growth and, consequently, larger grains but leads to forming voids within the entire interconnect structure. As the grain size of the metal increases, the resistance of the interconnect decreases resulting in lower-resistance wiring and a higher-performance integrated circuit. Additionally, the larger metal grains result in a more durable and reliable interconnect over the life of the integrated circuit. However, any metal void formed within the interconnect structure drastically degrades durability and reliability of interconnects.
Subsequently, the metal may be planarized to complete the formation of the interconnect, and in some cases, a dielectric cap layer may be formed over the interconnect. After formation of the interconnect, the interconnect can be exposed to subsequent anneal processes. Typically, the subsequent anneal process will be a batch furnace anneal at temperatures of approximately 300° C. to approximately 400° C., performed near the end of the wafer build. Additional anneals that could be performed after capping include pulsed laser anneals, which can be calibrated to achieved recrystallization, depending on the dielectric layers, metal pattern factor, and other specifics for a particular product.
Proper annealing of the metal is critical to suppress a reliability phenomenon known as electromigration. As current is passed through the metal conductor, the metal atoms may become physically displaced in the metal due to an effect known as electromigration. Electromigration refers to the transport of material caused by the movement of metal atoms in the conductor due to the momentum transfer between drift-current electrons and the metal atoms. At normal operating temperatures, electromigration occurs over an extended time (i.e., greater than the warranty period of the circuit) when the momentum of flowing electrons is transferred to a metal atom in the conductor lattice. However, at elevated temperatures, such as approximately 300° C. to approximately 400° C., such as are used during reliability testing, electromigration can be observed to occur in an accelerated time span of minutes or hours. This transfer of momentum causes the metal atoms to move from their original positions. As metal atoms spread to the exterior of the metal due to electromigration, the resulting tensile stresses in the metal cause voids to be created within the metal, resulting in interconnect and/or chip failures.